In Shakespeare's Julius Caesar, the Soothsayer warns Caesar to beware of the ides of March. "Ides" means the 15th of the month. This is just one of several signs in the early stages of the drama that warn of the chaotic, unnatural state of local politics in the play. Caesar, however, doesn't heed any of the warning signs and dismisses the Soothsayer as if he's speaking nonsense.
Of course, Ceasar ends up being assassinated on the 15th of March. Ironically, Caesar is superstitious, as shown when he arranges for his wife to be touched by Antony while Antony is racing, due to a myth that this may heal her inability to conceive. But he ignores the truly important omens and warnings that might have saved his life.
I believe that what you are asking about happens in Act I, Scene 2. What the soothsayer warns Julius Caesar about is something that you should have been asking yesterday, not today. What he tells Caesar is that Caesar must "beware the ides of March."
The reason I say you should have asked this yesterday is that yesterday was the ides of March. "Ides" was a term the Romans used to refer to the 15th of some months and the 13th of others.
The soothsayer was foretelling (as we later find out) that Caesar would be assassinated on that day.